HGH Reconstitution: IU, mg and Working-Concentration Calculations

Recombinant human growth hormone (somatropin) is supplied as a lyophilised (freeze-dried) powder. Reconstitution simply means dissolving that powder in a sterile diluent — commonly bacteriostatic water for injection or bacteriostatic 0.9% sodium chloride — to obtain a solution that can be measured and handled.

Research-use-only: This page covers the unit relationship and the laboratory arithmetic of preparing a reconstituted vial. It is provided for laboratory research use only and contains no dose, dosing schedule, or administration guidance of any kind. The quantities below are concentrations, not amounts to use.

The single most useful idea is that reconstitution produces a concentration: how much peptide sits in each millilitre of the finished solution. Everything else follows from that.

Growth hormone is quantified two ways, and confusing them is the most common error. Milligrams (mg) measure the mass of protein. International units (IU) measure biological potency, defined against a WHO International Standard.

For the current recombinant 22 kDa form (somatropin), the conversion is 3.0 IU per mg — i.e. 1 mg is about 3 IU. This is the specific activity assigned to the 2nd WHO International Standard for somatropin (NIBSC 98/574) and stated in its calibration study. So, for example, a vial labelled 10 IU contains roughly 3.3 mg of somatropin (10 divided by 3), and a 40 IU vial about 13.3 mg.

A historical note: older, pituitary-derived growth-hormone reference material carried a lower figure (on the order of about 2.6 IU/mg) before international standardisation settled on the 22 kDa recombinant form. That is a metrology history point, not a change in anything you measure today — use about 3 IU/mg for recombinant somatropin.

The working concentration of a reconstituted vial is just mass of peptide divided by volume of diluent added:

• A 10 IU vial (about 3.3 mg) reconstituted with 1 mL of bacteriostatic water gives 10 IU/mL (about 3.3 mg/mL).
• The same 10 IU vial in 2 mL gives 5 IU/mL. More diluent means a more dilute solution; the total IU in the vial is unchanged.
• A 40 IU vial (about 13.3 mg) in 2 mL gives 20 IU/mL — that is 40 IU divided by 2 mL, or equivalently 13.3 mg divided by 2 mL = about 6.65 mg/mL, which is 20 IU/mL at 3 IU/mg.

To move between the two units, multiply mg/mL by the 3 IU/mg factor to get IU/mL (or divide IU/mL by 3 to get mg/mL). These are statements about the concentration of the solution, nothing more. The reconstitution calculator performs this mass-to-volume-to-IU arithmetic for you.

This is the trap worth internalising. On a U-100 insulin syringe, the markings labelled “units” are a volume scale: U-100 means 100 units per millilitre, so 1 syringe unit = 0.01 mL. That “unit” has nothing to do with hormone activity.

An IU of HGH, by contrast, is a potency unit defined by the WHO International Standard. The two quantities merely share the word “unit.”

So a syringe unit and an IU of HGH are unrelated, and one never converts directly into the other. Only the concentration links them: if a solution is, say, 10 IU/mL, then 1 mL (100 syringe units) of it contains 10 IU — the IU content depends entirely on the concentration you reconstituted to, not on the syringe graduations.

The diluent typically carries the bacteriostatic preservative (e.g. benzyl alcohol or metacresol); the lyophilised powder itself contains no bacteriostat. Lyophilised somatropin is stored refrigerated at 2-8°C, and the reconstituted solution is likewise kept refrigerated and not frozen. For general laboratory handling of reconstituted peptides, see the peptide reconstitution and storage guide; this page focuses specifically on the HGH unit and concentration arithmetic.

Sources:

• Bristow AF, Jespersen AM. The Second International Standard for somatropin (recombinant DNA-derived human growth hormone): preparation and calibration. Biologicals. 2001;29(2):97-106. PMID 11580214. (defines specific activity as 3.0 IU/mg somatropin)
• NIBSC 98/574 — 2nd WHO International Standard for Somatropin (recombinant DNA-derived hGH): 3.0 IU/mg.
• NIBSC 80/505 — 1st WHO International Standard for Growth Hormone, Human, Pituitary (historical, about 2.6 IU/mg).
• FDA label, Zomacton (somatropin, rDNA): lyophilised powder, bacteriostatic-diluent reconstitution, 2-8°C storage.
• FDA label, Humulin R U-100: “U-100 insulin contains 100 units/mL” — basis for 1 syringe unit = 0.01 mL.

Continue across the HGH research cluster: the HGH research guide, the HGH molecular profile, HGH pharmacokinetics, and the HGH vs IGF-1 comparison. Reconstitute a vial with the reconstitution calculator. Research-grade vials: HGH 100IU kit, 240IU kit, and 400IU kit.